Rate responsive pacing system with QT sensor based on intrinsic QT data

ABSTRACT

There is provided a system and method of rate responsive pacing, having an intrinsic QT rate sensor. The system has the capability of sensing the QT interval of intrinsic cardiac signals and constructing from such intrinsic QT data a QT reference curve. The QT reference curve is used for comparison with beat-to-beat QT interval data, in order to provide sensor information for controlling pacing rate. The system of this invention preferably utilizes DSP circuitry for determining the occurrence of a Twave event and the timing of the Twave, from which the QT interval is calculated. The system also provides for compensation of any QT interval which is calculated following ventricular pacing, so that the QT sensor is operative at all times and throughout the entire rate range experienced by the patient. The system thereby provides an enhanced QT rate responsive pacing arrangement, and avoids the need of overdrive pacing in order to obtain QT reference data.

FIELD OF THE INVENTION

This invention relates to implantable rate responsive cardiac pacemakersystems and methods for rate control of cardiac pacing and, moreparticularly, rate responsive pacemaker systems that have the capabilityof adjusting the pacing rate as a function of QT interval.

BACKGROUND OF THE INVENTION

Implantable rate responsive pacemakers and other implantable medicaldevices that include rate responsive pacing have been in existence forsome years. In a normal patient who undertakes exercise or becomesinvolved in a stressful situation that calls for an increased heartrate, the patient's normal feedback mechanisms provide for the desiredincreased rate. However, for a variety of reasons the patient's normalfeedback mechanisms may be impaired. For a single chamber ventricularpacing system, or if the intrinsic atrial rate is unreliable, thepacemaker must carry out the task of determining the physiologicallydesired rate. Rate responsive pacing systems have the aim of providing asubstantially physiologic pacing rate by sensing one or more patientparameters and correlating desired rate with such parameters. Rateresponsive pacemakers are available as single chamber or dual chamberpacemakers.

A variety of sensors for determining a desired pacing rate are known inthe pacing art. The assignee of this invention has utilized the QTinterval (“QT”) as a parameter that is responsive to patient demand. TheQT interval is the time between contraction of the heart and therelaxation of the heart. This is manifested by electrical signals whichare measured by the implantable device when thedepolarization/repolarization waves pass the electrode system implantedwithin the patient's heart. The QT interval can be thought of as havinga rate dependent component and a stress dependent component. The ratedependent component causes the QT interval to shorten at higher rates asthe heart pumps more often and therefore reduces the time it stayscontracted. Conversely, the QT interval lengthens at lower rates. Thestress level, either physical or mental, also causes a QT intervalshortening at times of higher stress. Thus, when QT interval is measuredat any given rate, its value can be based both on the rate dependentcomponent and the stress component. In healthy hearts the QT intervalwill always correspond to the desired heart rate, and thus can be usedas a sensor for the metabolic need of the patient.

The QT sensor, to be effective, must have knowledge of the expected QTinterval corresponding to any given rate. This is generally accomplishedby maintaining within the pacemaker a QT reference curve, also referredto as a QT (RR) curve. If the QT interval at a given rate is shorterthan the corresponding reference curve value at that rate, thisindicates that the pacemaker should start pacing at a higher rate. Inprior art QT sensor systems, the QT interval has been measured onlyafter a ventricular pace, for the reason that it is difficult to sensethe T wave following an intrinsic QRS signal. In pacing, thedepolarization wave, or QRS complex, starts at the lead electrode orelectrodes where the stimulus is delivered, such that the “Q” time isactually the stimulus time. Further, the Twave that results from astimulus delivered through an electrode positioned adjacent the lead tipis reliably detected. However, in the case of an intrinsic QRST complex,the Twave comes from above the lead position, and can travel differentroutes, is less well defined and more difficult to sense. For thisreason, QT rate responsive pacemakers have relied upon obtaining QTmeasurements in response to delivered pacing pulses. These are actuallyStim-T values, measured from the time of delivery of the stimulus pulse.In such a system, when the patient's heart is naturally providingintrinsic QRST complexes, it is not possible to update or adjust the QT(RR) reference curve. In such circumstances, the QT rate responsivepacemaker historically has periodically initiated a pacing overdriveroutine, to capture the heart with higher rate pacing so as to obtain QT(RR) reference data. However, the overdrive feature is not desirable, asit paces the patient at a rate that isn't called for. Further, it is notable to provide reference data throughout the normal range of thepatient's intrinsic cardiac activity.

There are several problems with obtaining QT interval from intrinsicbeats. As noted, it is more difficult to detect the intrinsic T wave,which is not as well defined and has a smaller amplitude than does the Twave that follows an evoked QRS. The QT interval in the sensed case isshorter than that in the paced case. Further, the intrinsic ventricularcontraction might originate from an ectopic focus, which in turn cancause a deviation from the QT interval anticipated when the ventricularcontraction originates from the AV node. These problems must be overcomein any arrangement for sensing intrinsic QT intervals.

Examples of pacing systems incorporating a QT rate responsive pacing, aswell as techniques for adjusting the QT reference curve, are listed inTable 1 below. Also listed are patents showing the use of DSP technologyfor determination and classification of a sensed cardiac event.

TABLE 1 Patent No. Inventor(s) Issue Date 4,228,803 Rickards October,1980 4,972,834 Begemann et al Nov. 27, 1990 5,065,759 Begemann et alNov. 19, 1991 5,470,344 Begemann Nov. 28, 1995 5,978,711 Van Hove Nov.2, 1999 6,029,087 Wohlgemuth Feb. 22, 2000

All patents listed in Table 1 above are hereby incorporated by referenceherein in their respective entireties. As those of ordinary skill in theart will appreciate readily upon reading the Summary of the Invention,Detailed Description of the Preferred Embodiments and claims set forthbelow, many of the devices and methods disclosed in the patents of Table1 may be modified advantageously by using the teachings of the presentinvention.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved rate responsivepacing system that utilizes QT as the sensor parameter for determiningdesired pacing rate. It is a further object this invention to providesuch a system and method of pacing which obtains values of bothintrinsic and evoked QT, and constructs a QT (RR) reference curve acrossthe full range of the patient activity. A further object is to provide apacing system and method of rate control that utilizes a QT sensor anddoes not need to resort to overdrive pacing in order to get QTinformation.

In accordance with the above objects, there is provided a rateresponsive pacemaker and system of controlling pacing rate that utilizesQT interval data obtained from intrinsic heartbeats. The system andmethod of this invention accomplish the above object by utilizing DSP orequivalent circuit technology in combination with software to recognizeintrinsic Twaves and to determine QT intervals continuously whether thepatient is being paced or whether the patient has an intrinsicheartbeat. A preferred algorithm for reliably detecting the intrinsic Twave provides for integration of the sensed signal for a predeterminedtime window following the QRS complex, and determines the time of adetected T wave by timing either the maximum of the T wave slope or themaximum amplitude of the T wave. QT data obtained during pacing iscompensated to take into the account the difference between thestimulus-T wave time following the delivery of the pacing stimulus, andthe intrinsic QT time interval.

It is a further object of this invention to recognize any ventricularcontraction that is an ectopic focus, and to discriminate and not usesuch sensed ectopic beats for adjustment of the QT reference curve. Thisis accomplished by recognition of the different profile, or shape of theQRS complex of an ectopic beat, as compared to the normal depolarizationwave. DSP technology is suitably used for event classification andidentification, in order to discriminate ectopic beats.

The system and method of this invention can be utilized as well in apacemaker or implanted cardiac devise that utilizes more than one sensorfor determining a desired pacing rate. For example, U.S. Pat. No.5,065,759 illustrates a dual sensor pacemaker which utilizes both QT andactivity sensing. As stated in that patent, which is listed above inTable 1, the QT sensor supplements an activity sensor, and can provideimportant information when the patient is undergoing stress withoutbeing characterized by a level of activity that would indicate thedesired pacing rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic view of one embodiment of animplantable medical device that can be employed in the presentinvention.

FIG. 2 is a graphic representation of an implantable medical deviceinterconnected with a human or mammalian heart, illustrating the deviceconnector portion and the leads between the device and the heart.

FIG. 3 is a functional schematic diagram showing the primary constituentcomponents of an implantable medical device in accordance with anembodiment of this invention.

FIG. 4 is a graphic representation of an embodiment of this inventionshowing an implantable PCD device interconnected with a heart, thesystem of this embodiment providing pacing, cardio version anddefibrillation.

FIG. 5 is a functional schematic diagram of an implantable PCDembodiment of this invention.

FIG. 6 is a flow diagram of a routine cardiac cycle.

FIG. 7 shows three curves illustrating intrinsic Twaves and the mannerof determining the intrinsic QT interval.

FIG. 8 is a flow diagram illustrating the steps taken in determining theoccurrence of an intrinsic T wave event and for determining intrinsic QTinterval.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified schematic view of one embodiment of implantablemedical device (“IMD”) 10 of the present invention. IMD 10 shown in FIG.1 is a pacemaker comprising at least one of pacing and sensing leads 16and 18 attached to hermetically sealed enclosure 14 and implanted nearhuman or mammalian heart 8. Pacing and sensing leads 16 and 18 senseelectrical signals attendant to the depolarization and re-polarizationof the heart 8, and further provide pacing pulses for causingdepolarization of cardiac tissue in the vicinity of the distal endsthereof. Leads 16 and 18 may have unipolar or bipolar electrodesdisposed thereon, as is well known in the art. Examples of IMD 10include implantable cardiac pacemakers disclosed in U.S. Pat. No.5,158,078 to Bennett et al., U.S. Pat. No. 5,312,453 to Shelton et al.or U.S. Pat. No. 5,144,949 to Olson, all hereby incorporated byreference herein, each in its respective entirety.

FIG. 2 shows connector module 12 and hermetically sealed enclosure 14 ofIMD 10 located in and near human or mammalian heart 8. Atrial andventricular pacing leads 16 and 18 extend from connector header module12 to the right atrium and ventricle, respectively, of heart 8. Atrialelectrodes 20 and 21 disposed at the distal end of atrial pacing lead 16are located in the right atrium. Ventricular electrodes 28 and 29 at thedistal end of ventricular pacing lead 18 are located in the rightventricle.

FIG. 3 shows a block diagram illustrating the constituent components ofIMD 10 in accordance with one embodiment of the present invention, whereIMD 10 is pacemaker having a microprocessor-based architecture. IMD 10is shown as including activity sensor or accelerometer 11, which ispreferably a piezoceramic accelerometer bonded to a hybrid circuitlocated inside enclosure 14. Activity sensor 11 typically (although notnecessarily) provides a sensor output that varies as a function of ameasured parameter relating to a patient's metabolic requirements. Forthe sake of convenience, IMD 10 in FIG. 3 is shown with lead 18 onlyconnected thereto; similar circuitry and connections not explicitlyshown in FIG. 3 apply to lead 16.

IMD 10 in FIG. 3 is most preferably programmable by means of an externalprogramming unit (not shown in the Figures). One such programmer is thecommercially available Medtronic Model 9790 programmer, which ismicroprocessor-based and provides a series of encoded signals to IMD 10,typically through a programming head which transmits or telemetersradio-frequency (RF) encoded signals to IMD 10. Such a telemetry systemis described in U.S. Pat. No. 5,312,453 to Wyborny et al., herebyincorporated by reference herein in its entirety. The programmingmethodology disclosed in Wyborny et al's '453 patent is identifiedherein for illustrative purposes only. Any of a number of suitableprogramming and telemetry methodologies known in the art may be employedso long as the desired information is transmitted to and from thepacemaker.

As shown in FIG. 3, lead 18 is coupled to node 50 in IMD 10 throughinput capacitor 52. Activity sensor or accelerometer 11 is mostpreferably attached to a hybrid circuit located inside hermeticallysealed enclosure 14 of IMD 10. The output signal provided by activitysensor 11 is coupled to input/output circuit 54. Input/output circuit 54contains analog circuits for interfacing to heart 8, activity sensor 11,antenna 56 and circuits for the application of stimulating pulses toheart 8. The rate of heart 8 is controlled by software-implementedalgorithms stored in microcomputer circuit 58.

Microcomputer circuit 58 preferably comprises on-board circuit 60 andoff-board circuit 62. Circuit 58 may correspond to a microcomputercircuit disclosed in U.S. Pat. No. 5,312,453 to Shelton et al., herebyincorporated by reference herein in its entirety. On-board circuit 60preferably includes microprocessor 64, system clock circuit 66 andon-board RAM 68 and ROM 70. Off-board circuit 62 preferably comprises aRAM/ROM unit. On-board circuit 60 and off-board circuit 62 are eachcoupled by data communication bus 72 to digital controller/timer circuit74. Microcomputer circuit 58 may comprise a custom integrated circuitdevice augmented by standard RAM/ROM components.

Electrical components shown in FIG. 3 are powered by an appropriateimplantable battery power source 76 in accordance with common practicein the art. For the sake of clarity, the coupling of battery power tothe various components of IMD 10 is not shown in the Figures. Antenna 56is connected to input/output circuit 54 to permit uplink/downlinktelemetry through RF transmitter and receiver telemetry unit 78. By wayof example, telemetry unit 78 may correspond to that disclosed in U.S.Pat. No. 4,566,063 issued to Thompson et al., hereby incorporated byreference herein in its entirety, or to that disclosed in theabove-referenced '453 patent to Wyborny et al. It is generally preferredthat the particular programming and telemetry scheme selected permit theentry and storage of cardiac rate-response parameters. The specificembodiments of antenna 56, input/output circuit 54 and telemetry unit 78presented herein are shown for illustrative purposes only, and are notintended to limit the scope of the present invention.

Continuing to refer to FIG. 3, V_(REF) and Bias circuit 82 mostpreferably generates stable voltage reference and bias currents foranalog circuits included in input/output circuit 54. Analog-to-digitalconverter (ADC) and multiplexer unit 84 digitizes analog signals andvoltages to provide “real-time” telemetry intracardiac signals andbattery end-of-life (EOL) replacement functions. Operating commands forcontrolling the timing of IMD 10 are coupled by data bus 72 to digitalcontroller/timer circuit 74, where digital timers and counters establishthe overall escape interval of the IMD 10 as well as various refractory,blanking and other timing windows for controlling the operation ofperipheral components disposed within input/output circuit 54.

Digital controller/timer circuit 74 is preferably coupled to sensingcircuitry, including sense amplifier 88, peak sense and thresholdmeasurement unit 90 and comparator/threshold detector 92. Circuit 74 isfurther preferably coupled to electrogram (EGM) amplifier 94 forreceiving amplified and processed signals sensed by lead 18. Senseamplifier 88 amplifies sensed electrical cardiac signals and provides anamplified signal to peak sense and threshold measurement circuitry 90,which in turn provides an indication of peak sensed voltages andmeasured sense amplifier threshold voltages on multiple conductor signalpath 67 to digital controller/timer circuit 74. An amplified senseamplifier signal is then provided to comparator/threshold detector 92.By way of example, sense amplifier 88 may correspond to that disclosedin U.S. Pat. No. 4,379,459 to Stein, hereby incorporated by referenceherein in its entirety.

The electrogram signal provided by EGM amplifier 94 is employed when IMD10 is being interrogated by an external programmer to transmit arepresentation of a cardiac analog electrogram. See, for example, U.S.Pat. No. 4,556,063 to Thompson et al., hereby incorporated by referenceherein in its entirety. Output pulse generator 96 provides pacingstimuli to patient's heart 8 through coupling capacitor 98 in responseto a pacing trigger signal provided by digital controller/timer circuit74 each time the escape interval times out, an externally transmittedpacing command is received or in response to other stored commands as iswell known in the pacing art. By way of example, output amplifier 96 maycorrespond generally to an output amplifier disclosed in U.S. Pat. No.4,476,868 to Thompson, hereby incorporated by reference herein in itsentirety.

The specific embodiments of input amplifier 88, output amplifier 96 andEGM amplifier 94 identified herein are presented for illustrativepurposes only, and are not intended to be limiting in respect of thescope of the present invention. In the preferred embodiment, signalsfrom the patient's heart are coupled to an input channel chip shown at100, which chip provides outputs to the controller 74. The preferredembodiment of this chip incorporates DSP circuitry for identifyingectopic beats, as discussed in connection with FIG. 6 below, and as isfurther disclosed in U.S. Pat. No. 6,029,087, incorporated herein byreference. The specific embodiments of such circuits may not be criticalto practicing some embodiments of the present invention so long as theyprovide means for generating a stimulating pulse and are capable ofproviding signals indicative of natural or stimulated contractions ofheart 8.

In some preferred embodiments of the present invention, IMD 10 mayoperate in various non-rate-responsive modes, including, but not limitedto, DDD, DDI, VVI, VOO and VVT modes. In other preferred embodiments ofthe present invention, IMD 10 may operate in various rate-responsive,including, but not limited to, DDDR, DDIR, VVIR, VOOR and VVTR modes.Some embodiments of the present invention are capable of operating inboth non-rate-responsive and rate responsive modes. Moreover, in variousembodiments of the present invention IMD 10 may be programmablyconfigured to operate so that it varies the rate at which it deliversstimulating pulses to heart 8 only in response to one or more selectedsensor outputs being generated. Numerous pacemaker features andfunctions not explicitly mentioned herein may be incorporated into IMD10 while remaining within the scope of the present invention.

The present invention is not limited in scope to single-sensor ordual-sensor pacemakers, and is not limited to IMD's comprising activityor pressure sensors only. Nor is the present invention limited in scopeto single-chamber pacemakers, single-chamber leads for pacemakers orsingle-sensor or dual-sensor leads for pacemakers. Thus, variousembodiments of the present invention may be practiced in conjunctionwith more than two leads or with multiple-chamber pacemakers, forexample. At least some embodiments of the present invention may beapplied equally well in the contexts of single-, dual-, triple- orquadruple-chamber pacemakers or other types of IMD's. See, for example,U.S. Pat. No. 5,800,465 to Thompson et al., hereby incorporated byreference herein in its entirety, as are all U.S. Patents referencedtherein.

IMD 10 may also be a pacemaker-cardioverter-defibrillator (“PCD”)corresponding to any of numerous commercially available implantablePCD's. Various embodiments of the present invention may be practiced inconjunction with PCD's such as those disclosed in U.S. Pat. No.5,545,186 to Olson et al., U.S. Pat. No. 5,354,316 to Keimel, U.S. Pat.No. 5,314,430 to Bardy, U.S. Pat. No. 5,131,388 to Pless and U.S. Pat.No. 4,821,723 to Baker et al., all hereby incorporated by referenceherein, each in its respective entirety.

FIGS. 4 and 5 illustrate one embodiment of IMD 10 and a correspondinglead set of the present invention, where IMD 10 is a PCD. In FIG. 4, theventricular lead takes the form of leads disclosed in U.S. Pat. Nos.5,099,838 and 5,314,430 to Bardy, and includes an elongated insulativelead body 1 carrying three concentric coiled conductors separated fromone another by tubular insulative sheaths. Located adjacent the distalend of lead 1 are ring electrode 2, extendable helix electrode 3 mountedretractably within insulative electrode head 4 and elongated coilelectrode 5. Each of the electrodes is coupled to one of the coiledconductors within lead body 1. Electrodes 2 and 3 are employed forcardiac pacing and for sensing ventricular depolarizations. At theproximal end of the lead is bifurcated connector 6 which carries threeelectrical connectors, each coupled to one of the coiled conductors.Defibrillation electrode 5 may be fabricated from platinum, platinumalloy or other materials known to be usable in implantabledefibrillation electrodes and may be about 5 cm in length.

The atrial/SVC lead shown in FIG. 4 includes elongated insulative leadbody 7 carrying three concentric coiled conductors separated from oneanother by tubular insulative sheaths corresponding to the structure ofthe ventricular lead. Located adjacent the J-shaped distal end of thelead are ring electrode 9 and extendable helix electrode 13 mountedretractably within an insulative electrode head 15. Each of theelectrodes is coupled to one of the coiled conductors within lead body7. Electrodes 13 and 9 are employed for atrial pacing and for sensingatrial depolarizations. Elongated coil electrode 19 is provided proximalto electrode 9 and coupled to the third conductor within lead body 7.Electrode 19 preferably is 10 cm in length or greater and is configuredto extend from the SVC toward the tricuspid valve. In one embodiment ofthe present invention, approximately 5 cm of the right atrium/SVCelectrode is located in the right atrium with the remaining 5 cm locatedin the SVC. At the proximal end of the lead is bifurcated connector 17carrying three electrical connectors, each coupled to one of the coiledconductors.

The coronary sinus lead shown in FIG. 4 assumes the form of a coronarysinus lead disclosed in the above cited '838 patent issued to Bardy, andincludes elongated insulative lead body 41 carrying one coiled conductorcoupled to an elongated coiled defibrillation electrode 21. Electrode21, illustrated in broken outline in FIG. 4, is located within thecoronary sinus and great vein of the heart. At the proximal end of thelead is connector plug 23 carrying an electrical connector coupled tothe coiled conductor. The coronary sinus/great vein electrode 41 may beabout 5 cm in length.

Implantable PCD 10 is shown in FIG. 4 in combination with leads 1, 7 and41, and lead connector assemblies 23, 17 and 6 inserted into connectorblock 12. Optionally, insulation of the outward facing portion ofhousing 14 of PCD 10 may be provided using a plastic coating such asparylene or silicone rubber, as is employed in some unipolar cardiacpacemakers. The outward facing portion, however, may be left uninsulatedor some other division between insulated and uninsulated portions may beemployed. The uninsulated portion of housing 14 serves as a subcutaneousdefibrillation electrode to defibrillate either the atria or ventricles.Lead configurations other that those shown in FIG. 4 may be practiced inconjunction with the present invention, such as those shown in U.S. Pat.No. 5,690,686 to Min et al., hereby incorporated by reference herein inits entirety.

FIG. 5 is a functional schematic diagram of one embodiment ofimplantable PCD 10 of the present invention. This diagram should betaken as exemplary of the type of device in which various embodiments ofthe present invention may be embodied, and not as limiting, as it isbelieved that the invention may be practiced in a wide variety of deviceimplementations, including cardioverter and defibrillators which do notprovide anti-tachycardia pacing therapies.

IMD 10 is provided with an electrode system. If the electrodeconfiguration of FIG. 4 is employed, the correspondence to theillustrated electrodes is as follows. Electrode 25 in FIG. 5 includesthe uninsulated portion of the housing of PCD 10. Electrodes 25, 15, 21and 5 are coupled to high voltage output circuit 27, which includes highvoltage switches controlled by CV/defib control logic 29 via control bus31. Switches disposed within circuit 27 determine which electrodes areemployed and which electrodes are coupled to the positive and negativeterminals of the capacitor bank (which includes capacitors 33 and 35)during delivery of defibrillation pulses.

Electrodes 2 and 3 are located on or in the ventricle and are coupled tothe R-wave amplifier 37, which preferably takes the form of an automaticgain controlled amplifier providing an adjustable sensing threshold as afunction of the measured R-wave amplitude. A signal is generated onR-out line 39 whenever the signal sensed between electrodes 2 and 3exceeds the present sensing threshold.

Electrodes 9 and 13 are located on or in the atrium and are coupled tothe P-wave amplifier 43, which preferably also takes the form of anautomatic gain controlled amplifier providing an adjustable sensingthreshold as a function of the measured P-wave amplitude. A signal isgenerated on P-out line 45 whenever the signal sensed between electrodes9 and 13 exceeds the present sensing threshold. The general operation ofR-wave and P-wave amplifiers 37 and 43 may correspond to that disclosedin U.S. Pat. No. 5,117,824, by Keimel et al., issued Jun. 2, 1992, for“An Apparatus for Monitoring Electrical Physiologic Signals”, herebyincorporated by reference herein in its entirety.

Switch matrix 47 is used to select which of the available electrodes arecoupled to wide band (0.5-200 Hz) amplifier 49 for use in digital signalanalysis. Selection of electrodes is controlled by the microprocessor 51via data/address bus 53, which selections may be varied as desired.Signals from the electrodes selected for coupling to bandpass amplifier49 are provided to multiplexer 55, and thereafter converted to multi-bitdigital signals by A/D converter 57, for storage in random access memory59 under control of direct memory access circuit 61. Microprocessor 51may employ digital signal analysis techniques to characterize thedigitized signals stored in random access memory 59 to recognize andclassify the patient's heart rhythm employing any of the numerous signalprocessing methodologies known to the art.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies, and, for purposes ofthe present invention may correspond to circuitry known to those skilledin the art. The following exemplary apparatus is disclosed foraccomplishing pacing, cardioversion and defibrillation functions. Pacertiming/control circuitry 63 preferably includes programmable digitalcounters which control the basic time intervals associated with DDD,VVI, DVI, VDD, AAI, DDI and other modes of single and dual chamberpacing well known to the art. Circuitry 63 also preferably controlsescape intervals associated with anti-tachyarrhythmia pacing in both theatrium and the ventricle, employing any anti-tachyarrhythmia pacingtherapies known to the art.

Intervals defined by pacing circuitry 63 include atrial and ventricularpacing escape intervals, the refractory periods during which sensedP-waves and R-waves are ineffective to restart timing of the escapeintervals and the pulse widths of the pacing pulses. The durations ofthese intervals are determined by microprocessor 51, in response tostored data in memory 59 and are communicated to pacing circuitry 63 viaaddress/data bus 53. Pacer circuitry 63 also determines the amplitude ofthe cardiac pacing pulses under control of microprocessor 51.

During pacing, escape interval counters within pacer timing/controlcircuitry 63 are reset upon sensing of R-waves and P-waves as indicatedby a signals on lines 39 and 45, and in accordance with the selectedmode of pacing on time-out trigger generation of pacing pulses by paceroutput circuitry 65 and 67, which are coupled to electrodes 9, 13, 2 and3. Escape interval counters are also reset on generation of pacingpulses and thereby control the basic timing of cardiac pacing functions,including anti-tachyarrhythmia pacing. The durations of the intervalsdefined by escape interval timers are determined by microprocessor 51via data/address bus 53. The value of the count present in the escapeinterval counters when reset by sensed R-waves and P-waves may be usedto measure the durations of R—R intervals, P—P intervals, P-R intervalsand R-P intervals, which measurements are stored in memory 59 and usedto detect the presence of tachyarrhythmias.

Microprocessor 51 most preferably operates as an interrupt drivendevice, and is responsive to interrupts from pacer timing/controlcircuitry 63 corresponding to the occurrence sensed P-waves and R-wavesand corresponding to the generation of cardiac pacing pulses. Thoseinterrupts are provided via data/address bus 53. Any necessarymathematical calculations to be performed by microprocessor 51 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 63 take place following such interrupts.

Detection of atrial or ventricular tachyarrhythmias, as employed in thepresent invention, may correspond to tachyarrhythmia detectionalgorithms known in the art. For example, the presence of an atrial orventricular tachyarrhythmia may be confirmed by detecting a sustainedseries of short R—R or P—P intervals of an average rate indicative oftachyarrhythmia or an unbroken series of short R—R or P—P intervals. Thesuddenness of onset of the detected high rates, the stability of thehigh rates, and a number of other factors known in the art may also bemeasured at this time. Appropriate ventricular tachyarrhythmia detectionmethodologies measuring such factors are described in U.S. Pat. No.4,726,380 issued to Vollmann, U.S. Pat. No. 4,880,005 issued to Pless etal. and U.S. Pat. No. 4,830,006 issued to Haluska et al., allincorporated by reference herein, each in its respective entirety. Anadditional set of tachycardia recognition methodologies is disclosed inthe article “Onset and Stability for Ventricular TachyarrhythmiaDetection in an Implantable Pacer-Cardioverter-Defibrillator” by Olsonet al., published in Computers in Cardiology, Oct. 7-10, 1986, IEEEComputer Society Press, pages 167-170, also incorporated by referenceherein in its entirety. Atrial fibrillation detection methodologies aredisclosed in Published PCT Application Ser. No. US92/02829, PublicationNo. WO92/18198, by Adams et al., and in the article “AutomaticTachycardia Recognition”, by Arzbaecher et al., published in PACE,May-June, 1984, pp. 541-547, both of which are incorporated by referenceherein in their entireties.

In the event an atrial or ventricular tachyarrhythmia is detected and ananti-tachyarrhythmia pacing regimen is desired, appropriate timingintervals for controlling generation of anti-tachyarrhythmia pacingtherapies are loaded from microprocessor 51 into the pacer timing andcontrol circuitry 63, to control the operation of the escape intervalcounters therein and to define refractory periods during which detectionof R-waves and P-waves is ineffective to restart the escape intervalcounters.

Alternatively, circuitry for controlling the timing and generation ofanti-tachycardia pacing pulses as described in U.S. Pat. No. 4,577,633,issued to Berkovits et al. on Mar. 25, 1986, U.S. Pat. No. 4,880,005,issued to Pless et al. on Nov. 14, 1989, U.S. Pat. No. 4,726,380, issuedto Vollmann et al. on Feb. 23, 1988 and U.S. Pat. No. 4,587,970, issuedto Holley et al. on May 13, 1986, all of which are incorporated hereinby reference in their entireties, may also be employed.

In the event that generation of a cardioversion or defibrillation pulseis required, microprocessor 51 may employ an escape interval counter tocontrol timing of such cardioversion and defibrillation pulses, as wellas associated refractory periods. In response to the detection of atrialor ventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, microprocessor 51 activates cardioversion/defibrillation controlcircuitry 29, which initiates charging of the high voltage capacitors 33and 35 via charging circuit 69, under the control of high voltagecharging control line 71. The voltage on the high voltage capacitors ismonitored via VCAP line 73, which is passed through multiplexer 55 andin response to reaching a predetermined value set by microprocessor 51,results in generation of a logic signal on Cap Full (CF) line 77 toterminate charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse is controlled by pacertiming/control circuitry 63. Following delivery of the fibrillation ortachycardia therapy microprocessor 51 returns the device to a cardiacpacing mode and awaits the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization.

Several embodiments of appropriate systems for the delivery andsynchronization of ventricular cardioversion and defibrillation pulsesand for controlling the timing functions related to them are disclosedin U.S. Pat. No. 5,188,105 to Keimel, U.S. Pat. No. 5,269,298 to Adamset al. and U.S. Pat. No. 4,316,472 to Mirowski et al., herebyincorporated by reference herein, each in its respective entirety. Anyknown cardioversion or defibrillation pulse control circuitry isbelieved to be usable in conjunction with various embodiments of thepresent invention, however. For example, circuitry controlling thetiming and generation of cardioversion and defibrillation pulses such asthat disclosed in U.S. Pat. No. 4,384,585 to Zipes, U.S. Pat. No.4,949,719 to Pless et al., or U.S. Pat. No. 4,375,817 to Engle et al.,all hereby incorporated by reference herein in their entireties, mayalso be employed.

Continuing to refer to FIG. 5, delivery of cardioversion ordefibrillation pulses is accomplished by output circuit 27 under thecontrol of control circuitry 29 via control bus 31. Output circuit 27determines whether a monophasic or biphasic pulse is delivered, thepolarity of the electrodes and which electrodes are involved in deliveryof the pulse. Output circuit 27 also includes high voltage switcheswhich control whether electrodes are coupled together during delivery ofthe pulse. Alternatively, electrodes intended to be coupled togetherduring the pulse may simply be permanently coupled to one another,either exterior to or interior of the device housing, and polarity maysimilarly be pre-set, as in current implantable defibrillators. Anexample of output circuitry for delivery of biphasic pulse regimens tomultiple electrode systems may be found in the above cited patent issuedto Mehra and in U.S. Pat. No. 4,727,877, hereby incorporated byreference herein in its entirety.

An example of circuitry which may be used to control delivery ofmonophasic pulses is disclosed in U.S. Pat. No. 5,163,427 to Keimel,also incorporated by reference herein in its entirety. Output controlcircuitry similar to that disclosed in U.S. Pat. No. 4,953,551 to Mehraet al. or U.S. Pat. No. 4,800,883 to Winstrom, both incorporated byreference herein in their entireties, may also be used in conjunctionwith various embodiments of the present invention to deliver biphasicpulses.

Alternatively, IMD 10 may be an implantable nerve stimulator or musclestimulator such as that disclosed in U.S. Pat. No. 5,199,428 to Obel etal., U.S. Pat. No. 5,207,218 to Carpentier et al. or U.S. Pat. No.5,330,507 to Schwartz, or an implantable monitoring device such as thatdisclosed in U.S. Pat. No. 5,331,966 issued to Bennet et al., all ofwhich are hereby incorporated by reference herein, each in itsrespective entirety. The present invention is believed to find wideapplication to any form of implantable electrical device for use inconjunction with electrical leads.

FIG. 6 illustrates the overall routine for determining the desired QTsensor rate on a cycle-by-cycle basis. The routine is run following eachVevent, sense or pace, starting at 200. A Vsense represents a detected Rwave, and is determined in a known manner such as is disclosed inreferenced U.S. Pat. No. 6,029,087. Whether a sense or a pace, the R—Rinterval of the cycle is timed out and stored. At 200 it is alsodetermined whether the event has indeed been a sense. If a ventricularsense has been detected, the routine goes to block 202 and assigns tothe DSP circuitry 100 the task of determining if the sense was AV inorigin or ectopic. At 203, the routine branches to 212 if it has beendetermined that it was an ectopic sense. In this case, as discussedabove, the QT interval cannot be accurately determined, and no QT datais stored before the routine exits.

If, at 203, it has been confirmed that the sense was intrinsic AV inorigin, then at 205 the QT interval is measured. This is done in accordwith the steps set forth in more detail in connection with FIG. 8. AfterQT interval has been determined, at 206 the QT interval data is placedinto a database together with the rate for construction of a QTreference curve. At 208, it is determined whether QT data has beengathered for a predetermined period of time, indicated as one week. Thisis a programmable variable, and can be set by the physician as either aperiod of elapsed time or a predetermined number of patient cardiaccycles. If sufficient data has not been gathered, the routine goes to210 and calculates QT sensor rate in the conventional manner, i.e., bycomparing the measured QT interval to the QT reference curve. Such acalculation is illustrated in the prior art, e.g., U.S. Pat. Nos.4,972,834 and 5,065,759. Following this, the QT sensor rate is utilizedfor control of pacing when and as the pacing is required.

If at block 208 sufficient data has been obtained and stored, than theroutine branches to 209 and calculates a new QT reference curve.Techniques for adjusting QT reference curve are shown in the patentliterature. See, for example, U.S. Pat. No. 4,972,834.

If the Vevent has been a Vpace, at 200 the routine branches to 215 andmeasures the QT interval (Stim-T) in the standard way. At 216, the QTinterval is compensated for pacing. As discussed above, the QT intervalin the event of pacing is the stimulus-T interval, and is longer thanthe intrinsic QT interval. For most patients, the difference will besubstantially a constant throughout the pacing range, such that thecompensation at block 216 may be performed simply by subtracting apredetermined number of milliseconds from the measured QT interval. Thecompensation adjustment is a programmable variable. The compensated QTdata may be stored as part of this step, as part of the QT database.Following compensation, the compensated QT interval is used at block 210to calculate the QT sensor rate. It is to be noted that, while notshown, the compensated QT interval can also be stored and used tocalculate the QT reference curve, in the same manner as done at blocks208 and 209.

In the operation of the routine of FIG. 6, there is no rate limit on thecollection of QT data. Thus, control of pacing rate is limited to a highrate limit, as is conventional in any rate responsive pacing system.However, it is desirable to obtain and store QT data at higher rates,and this routine enables measuring of the QT interval at rates above thesensor high rate limit. The gathered QT data is stored for diagnosticpurposes, and can be compressed into histogram form or another form forstorage until downloaded to an external programmer for analysis.

FIG. 7 shows a series of curves that illustrate the preferred techniquefor determining QT interval for intrinsic cardiac signals. The top curveillustrates an unfiltered signal, such as is provided followingamplification. The second curve shows a filtered signal, such as can beprovided by the DSP circuitry illustrated in U.S. Pat. No. 6,029,087,incorporated herein by reference in its entirety. The timing of the Rportion of the QRS wave is indicated by R. The Twave window isidentified as T window and can be programmed to start at a predetermineddelay following the peak R signal. The DSP circuitry integrates thetotal area of the curve following the start of the window, andidentifies a T wave when the total integration becomes equal to apredetermined threshold. In the illustration of the middle curve, the Twave is identified as a T wave event by the vertical line marked T. Notethat this signals detection of a T wave, but does not provide the timingof the T wave. The bottom curve of FIG. 8 illustrates the Twave slope,which also is available from the DSP circuitry in accord with the abovereferenced patent. The time of maximum slope during the Twave window isused as the time of occurrence of the T wave, indicated as T_(t). The QTinterval is shown as the time between the R wave and T_(t).

FIG. 8 shows a flow diagram for measuring QT interval, which is step 205in FIG. 6. At the time of the Vevent, whether a pace or a sense, a timeris initiated. Thus, at the start of the routine of FIG. 8, t=0. Theclock continues to run during the course of executing this routine. At225, the microprocessor sends control signals to the DSP circuitry, togenerate the filtered Twave as seen in the middle curve of FIG. 7, aswell as the Twave slope curve as seen in the bottom curve of FIG. 7.When time progresses to T_(w) as shown in the middle curve of FIG. 7,integration of the filtered Twave is initiated, as illustrated at 228.The routine then proceeds to watch the value of the integral, andcompares it to an integral threshold indicated at 231 as Ith. If theintegral of the signal becomes greater than Ith, this indicatesdetection of a T wave. Then at 232 the routine finds the maximum slope,and gets the time T_(t) of the occurrence of the maximum slope. Thistime is in fact QT, and at block 234 QT is set equal to T_(t) andstored. However, if the integral never reaches Ith at 231, the routineexits to 235 and sets QT as unknown.

In the preferred embodiment of FIG. 8, the integral of the T wave isused as a measure of the T wave in order to detect the occurrence of a Twave. It is to be noted that other techniques may be used, within thescope of the invention, to detect the presence of a T wave. For example,morphology analysis may be utilized to obtain a “measure” of the T wave,as that term is used in the claims. Further, while the time of themaximum T wave slope signal is used in the preferred embodiment foridentification of the T wave time, other measures of the T wave may beused.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, therefore, that other expedientsknown to those skilled in the art or disclosed herein, may be employedwithout departing from the invention or the scope of the appendedclaims. For example, the present invention is not limited to the use ofDSP circuitry. The references to DSP do not exclude structures oroperations that include some conventional analog circuitry. The presentinvention is also not limited to any particular combination of hardwareand software per se, but may find further application with any form ofsoftware supplementing hardware. For example, other software embodimentsthat achieve the ability to efficiently store and manipulate the data,and analyze the T wave portion of the ventricular signal, are within thescope of the invention. The present invention further includes withinits scope methods of making and using the QT sensor-driven pacing systemdescribed hereinabove.

In the claims, means-plus-function clauses are intended to cover thestructures described herein as performing the recited function and notonly structural equivalents but also equivalent structures. Thus,although a nail and a screw may not be structural equivalents in that anail employs a cylindrical surface to secure wooden parts together,whereas a screw employs a helical surface, in the environment offastening wooden parts a nail and a screw are equivalent structures.

What is claimed is:
 1. A rate responsive pacing system having apacemaker device and a lead system interconnecting said device and apatient's heart, said device having a pulse generator for generatingpacing pulses, sensing means for sensing cardiac signals connected fromsaid pulse generator to deliver pacing pulses at said sensor rate, saidsensor means comprising: algorithm means for calculating a measure ofthe T wave portion of a received intrinsic cardiac signal; comparingmeans for comparing said measure with a predetermined criterion todetect the occurrence of the T wave portion of said signal; data meansfor determining and storing intrinsic QT interval data and heart ratedata for each received intrinsic signal for which a T wave has beendetected; and reference means for setting a QT reference curve as afunction of said stored intrinsic QT interval and rate data, whereinsaid sensor means comprises high rate limit means for setting a maximumsensor rate, and wherein said algorithm means, said comparing means andsaid state means are operative at rates above said maximum sensor rate.2. The system as described in claim 1, wherein said algorithm meanscomprises integration means for integrating said T wave portion, andsaid comparing means comprises programmable storage means for storing athreshold integral value that represents a valid T wave sense.
 3. Thesystem as described in claim 2, comprising Vpace means for determiningwhen a ventricular pace has been delivered, second QT data means fordetermining evoked QT interval data and heart rate data, andcompensating means for compensating said evoked QT interval data toaccount for the difference between intrinsic and invoked QT intervals.4. The system as described in claim 3, wherein said second QT data meanscomprises slope means for detecting the occurrence of maximum T waveslope and for determining the time of said slope as the time of T waveoccurrence.
 5. A rate responsive pacing system having a pacemaker deviceand a lead system interconnecting said device and a patient's heart,said device having a pulse generator for generating pacing pulses,sensing means for sensing cardiac signals connected from said pulsegenerator to deliver pacing pluses at said sensor rate, said sensormeans comprising: algorithm means for calculating a measure of the Twave portion of a received intrinsic cardiac signal; comparing means forcomparing said measure with a predetermined criterion to detect theoccurrence of the T wave portion of said signal; data means fordetermining and storing intrinsic QT interval data and heart rate datafor each received intrinsic signal for which a T wave has been detected;reference means for setting a QT reference curve as a function of saidstored intrinsic QT interval and rate data; and second algorithm meansfor determining and storing values of evoked QT interval for evokedcardiac signals, and wherein said rate control means comprisescalculating means for calculating sensor rate following a ventricularevent as a function of said QT reference curve and the evoked QTinterval.
 6. The system as described in claim 5, wherein said secondalgorithm means comprises compensation means for compensating eachdetermined value of evoked QT interval to correspond to an intrinsic QTinterval at the same rate.
 7. The system as described in claim 6,wherein said compensation means comprises ms means for adjusting eachdetermined value of evoked QT interval by a predetermined ms value,thereby compensating for the difference in measurements of intrinsic andevoked QT intervals.
 8. The system as described in claim 5, wherein saiddata means has storage for storing data for a predetermined time, andwhereas said reference means comprises means for calculating said QTreference curve when said data has been stored for said predeterminedtime.
 9. The system as described in claim 8, wherein said referencemeans comprises time means for calculating said reference curve whendata has been stored for a time period in the range of one day to onemonth.
 10. A rate responsive pacing system having a pacemaker device anda lead system interconnecting said device and a patient's heart, saiddevice having a pulse generator for generating pacing pulses, sensingmeans for sensing cardiac signals connected from said pulse generator todeliver pacing pulses at said sensor rate, said sensor means comprising:algorithm means for calculating a measure of the T wave portion of areceived intrinsic cardiac signal; comparing means for comparing saidmeasure with a predetermined criterion to detect the occurrence of the Twave portion of said signal; data means for determining and storingintrinsic QT interval data and heart rate data for each receivedintrinsic signal for which a T wave has been detected; reference meansfor setting a QT reference curve as a function of said stored intrinsicQT interval and rate data; and ectopic means for determining when asensed cardiac signal represents an ectopic beat, and for setting saidsensor rate as unknown for any such ectopic beat.
 11. The system asdescribed in claim 10, wherein said ectopic means comprises DSPcircuitry for analyzing the shape of a sensed cardiac signal.
 12. A rateresponsive pacing system having a pacemaker device and a lead systeminterconnecting said device and a patient's heart, said device having apulse generator for generating pacing pulses, sensing means for sensingcardiac signals connected from said heart by said lead system, sensormeans for determining from received cardiac signals a sensor pacingrate, and rate control means for controlling said pulse generator todeliver pulses at said sensor rate, said sensor means comprising: ratemeans for determining values of the patient's heart rate each cardiaccycle; intrinsic means for obtaining QT intervals of intrinsicventricular signals; stim means for obtaining evoked QT intervals ofstimulated ventricular signals; compensating means for compensating saidevoked QT intervals to obtain compensated QT intervals that correspondto intrinsic QT intervals; reference means for constructing and storinga QT reference curve from said intrinsic QT intervals and said ratevalues; and calculating means for determining said sensor pacing ratefrom each respective QT and compensated QT interval.
 13. The system asdescribed in claim 12, wherein said compensating means comprises astored constant value and an adjustment algorithm for adjusting theevoked QT by said stored value.
 14. The system as described in claim 12,comprising differential means for storing data representing thedifference between intrinsic QT and evoked QT as a function of rate. 15.The system as described in claim 12, wherein said intrinsic meanscomprises algorithm means for calculating a measure of the T waveportion of a received intrinsic signal; comparing means for comparingsaid measure with a predetermined criterion to detect the occurrence ofan intrinsic T wave; and timing means for determining the QT time of anintrinsic signal having a detected T wave.
 16. The system as describedin claim 15, wherein said algorithm means comprising integrating meansfor integrating said signal within a predetermined time following the Rportion of said signal.
 17. The system as described in claim 16, whereinsaid comparing means comprises threshold means for comparing saidintegration with a predetermined threshold value.
 18. The system asdescribed in claim 16, wherein said timing means comprises means forobtaining the slope of said T wave portion and for determining the peakvalue of said slope.
 19. A method of controlling the pacing rate of apacing system implanted in a patient, comprising: sensing intrinsicpatient ventricular signals; determining from each said sensed signalthe time of the R wave and calculating and storing the rate of each lastcardiac cycle; analyzing each said sensed signal within a predeterminedtime window following the R wave time to determine at least one measureof said signal; comparing said measure to at least one criterion anddetecting the occurrence of a T wave as a function said comparing; forsaid detected T wave, determining the time between said R wave and saidT wave and storing said time as a value of intrinsic QT; storing a QTreference curve that provides reference values QT as a function of rate;comparing each determined value of QT to said reference curve anddetermining therefrom an indicated pacing rate; controlling the rate ofpacing as a function of said indicated pacing rate; storing values ofintrinsic QT and rate corresponding to each detected T wave, andperiodically recalculating said QT reference curve as a function of saidstored values; and determining when a sensed ventricular signalrepresents an ectopic beat, and inhibiting determination and storing ofa QT value for any such ectopic beat.
 20. The method as described inclaim 19, comprising determining the occurrence of a predeterminedproperty of a said detected T wave, and storing the time between said Rwave and said occurrence as the value of QT.
 21. The method as describedin claim 20, comprising obtaining the slope of said sensed signalfollowing the R wave, and determining the time of the maximum of saidslope.
 22. The method as described in claim 21, comprising initiating atime window a predetermined number of ms following the time of said Rwave.
 23. The method as described in claim 22, comprising integratingsaid sensed signal at the start of said time window.
 24. The method asdescribed in claim 23, comprising storing a threshold integration value,and continuing said integrating until the intergral reaches saidthreshold or said time window times out.
 25. The method as described inclaim 24, comprising detecting a T wave when said integral reaches saidthreshold.
 26. A method of controlling the pacing rate of a pacingsystem implanted in a patient, comprising: sensing intrinsic patientventricular signals; determining from each said sensed signal the timeof the R wave and calculating and storing the rate of each last cardiaccycle; analyzing each said sensed signal within a predetermined timewindow following the R wave time to determine at least one measure ofsaid signal; comparing said measure to at least one criterion anddetecting the occurrence of a T wave as a function of said comparing;for a said detected T wave, determining the time between said R wave andsaid T wave and storing said time as a value of intrinsic QT; storing aQT reference curve that provides reference values of QT as a function ofrate; comparing each determined value of QT to said reference curve anddetermining therefrom an indicated pacing rate; controlling the rate ofpacing as a function of said indicated pacing rate; and determining whenthere has been a delivered ventricular pace, determining a value ofevoked QT for the evoked QRST following a delivered ventricular pace,and compensating said evoked QT to correct for the longer time of anevoked QT compared to an intrinsic QT.
 27. The method as described inclaim 26, comprising storing values of rate and compensated QT followinga delivered pace.
 28. The method as described in claim 26, comprisingperiodically recalculating said QT reference curve as a function ofstored values of intrinsic and compensated QT.
 29. The method asdescribed in claim 28, comprising storing values of QT and correspondingvalues of rate for ventricular events across the patient's ratespectrum, and downloading said QT and rate data to an external site foranalysis.
 30. A method of controlling the pacing rate of a pacing systemimplanted in a patient, comprising: sensing intrinsic patientventricular signals; determining from each said sensed signal the timeof the R wave and calculating and storing the rate of each last cardiaccycle; analyzing each said sensed signal within a predetermined timewindow following the R wave time to determine at least one measure ofsaid signal; comparing said measure to at least one criterion anddetecting the occurrence of a T wave as a function of said comparing:for a said detected T wave, determining the time between said R wave andsaid T wave and storing said time as a value of intrinsic QT; storing aQT reference curve that provides reference values of QT as a function ofrate; comparing each determined value of QT to said reference curve anddetermining therefrom an indicated pacing rate; controlling the rate ofpacing as a function of said indicated pacing rate; and determiningvalues of evoked QT for evoked ventricular events, compensating saidvalues of evoked QT to substantially equate them to intrinsic values,and determining indicated pacing rate by comparing any determined valueof QT to said QT reference curve.
 31. The method as described in claim30, comprising storing values of intrinsic and evoked QT andcorresponding rate values, and periodically recalculating said QTreference curve as a function of said stored values.
 32. An implantablepacing system for pacing a patient, said system having a pacinggenerator that generates pacing pulses at a controllable pacing rate anda rate control subsystem for controlling pacing rate, said subsystemcomprising: signal circuitry for receiving and processing intrinsicpatient ventricular signals; R wave means for detecting the occurrenceof an R wave; T wave means for determining the occurrence of a T wave,said T wave means comprising measure means for obtaining a measure ofeach said intrinsic signal over a period of time following a detected Rwave; QT means for determining the intrinsic QT interval of an intrinsicsignal having a detected R wave and a detected T wave; rate control mansfor controlling pacing rate as a function of determined QT intervals;DSP circuitry for providing the slope of said intrinsic signal followinga detected T wave, and peak means for identifying the maximum value ofsaid slope as the time of a T wave; storage means for storing values ofintrinsic QT and rate for each detected T wave; reference means forcalculating a QT reference curve as a function of said stored values;and DSP means for determining when a sensed ventricular signalrepresents an ectopic beat, and inhibiting means for inhibitingdetermining of a QT value for any such ectopic beat, wherein said T wavemeans further comprises storage that stores predetermined T wavecriteria, and comparing means for comparing said measure with saidcriteria, and wherein said measure means comprises window means forinitiating a window at a predetermined time following a detected R wave,and integrating means for integrating each said received intrinsicsignal starting at the initiation of a said window.
 33. An implantablepacing system for pacing a patient, said system having a pacinggenerator that generates pacing pulses at a controllable pacing rate anda rate control subsystem for controlling pacing rate, said subsystemcomprising: signal circuitry for receiving and processing intrinsicpatient ventricular signals; R wave means for detecting the occurrenceof an R wave; T wave means for determining the occurrence of a T wave,said T wave means comprising measure means for obtaining a measure ofeach said intrinsic signal over a period of time following a detected Rwave; QT means for determining the intrinsic QT interval of an intrinsicsignal having a detected R wave and a detected T wave; rate control mansfor controlling pacing rate as a function of determined QT intervals;means for determining the QT interval of evoked ventricular beats andfor storing data representative of said evoked QT intervals; and meansfor compensating values of QT interval of evoked beats to substantiallyequate such values with intrinsic values of QT interval.
 34. The systemas described in claim 33, comprising storage means for storingdetermined intrinsic and evoked QT intervals, and reference means forconstructing a QT reference curve from said stored values.